300 I The Process of Evolution 



of insects may be fragmented into distinct biological species has 

 indeed been overestimated. A study of North American butterflies 

 showed that less than one-half of the genera could be divided neatly 

 into groups of entities that might correspond to biological species. 

 In retrospect, these conclusions are not very surprising. The very 

 nature of the biological-species definition makes its use in practice 

 impossible. The accepted test of conspecificity (or lack of it) is 

 what happens ( or would happen ) when two forms occur together 

 in nature. Even when, conveniently, two forms are sympatric, studies 

 may reveal some level of partial interbreeding and make a firm 

 decision impossible. In such a situation, the question revolves around 

 whether or not swamping will overcome any tendency toward isola- 

 tion caused by a selective disadvantage of the hybrids. This problem 

 has been discussed in the chapter on differentiation of populations; 

 since the environmental conditions are certain to change, the only 

 way to answer the question of what will happen is to wait and see. 

 In allopatric entities the problem is complicated by the idea of 

 "potential interbreeding." One must predict the courses of popula- 

 tions if they approach each other and the events at their hypothetical 

 meeting under assumed conditions. Then, if the formation of hybrids 

 is postulated, the fitness or viability of the hybrid population must 

 be estimated. Unfortunately, fitness and viability are very difficult 

 to estimate for a population of Drosophih in a bottle, let alone for 

 a hypothetical population resulting from presumed hybridization of 

 populations that have traveled unknown routes to an assumed lo- 

 cality where postulated conditions prevail. 



Laboratory tests of interbreeding potential, while yielding valu- 

 able information, are not considered definitive. Two kinds of mice, 

 Peromyscus leucopus and P. gossypiamts, will hybridize in the 

 laboratory, but where they occur together naturally in the Great 

 Dismal Swamp of Virginia they remain distinct. Laboratory hybrids 

 between northern and southern populations of Rana pipiens do not 

 develop properly. As discussed in Chap. 10, these northern and 

 southern frogs are connected by a long chain of intermediate popula- 

 tions. Should these intermediate populations suddenly become 

 extinct, would the two terminal groups represent two "good" bio- 

 logical species? The question cannot be answered because the 

 biological-species concept has no operational definition. There is no 

 set of operations by which a value can be assigned to each entity: in 

 this case, either "good species" or "not good species." It is con- 

 ceivable that, if the two terminal populations approached one an- 

 other naturally, selection might alter them so that they would 

 interbreed freely on meeting; then again it might not. 



